Macro-scale structures formed from concentrically-layered nanoscale or microscale fibers (“core-sheath fibers”) are useful in a wide range of applications including drug delivery, tissue engineering, nanoscale sensors, self-healing coatings, and filters. On a commercial scale, the most commonly used techniques for manufacturing core-sheath fibers are extrusion, fiber spinning, melt blowing, and thermal drawing. None of these methods, however, are ideally suited to producing drug-loaded core-sheath fibers, as they all utilize high temperatures which may be incompatible with thermally labile materials such as drugs or polypeptides. Additionally, fiber spinning, extrusion and melt-blowing are most useful in the production of fibers with diameters greater than ten microns.
Core-sheath fibers can be produced by electrospinning, in which an electrostatic force is applied to a polymer solution to form very fine fibers. Conventional electrospinning methods utilize a charged needle to supply a polymer solution, which is then ejected in a continuous stream toward a grounded collector. After removal of solvents by evaporation, a single long polymer fiber is produced. Core-sheath fibers have been produced using emulsion-based electrospinning methods, which exploit surface energy to produce core-sheath fibers, but which are limited by the relatively small number of polymer mixtures that will emulsify, stratify, and electrospin. Core-sheath fibers have also been produced using coaxial electrospinning, in which concentric needles are used to eject different polymer solutions: the innermost needle ejects a solution of the core polymer, while the outer needle ejects a solution of the sheath polymer. This method is particularly useful for fabrication of core-sheath fibers for drug delivery in which the drug-containing layer is confined to the center of the fiber and is surrounded by a drug-free layer. However, both emulsion and coaxial electrospinning methods can have relatively low throughput, and are not ideally suited to large-scale production of core-sheath fibers. To increase throughput, coaxial nozzle arrays have been utilized, but such arrays pose their own challenges, as separate nozzles may require separate pumps, the multiple nozzles may clog, and interactions between nozzles may lead to heterogeneity among the fibers collected. Another means of increasing throughput, which utilizes a spinning drum immersed in a bath of polymer solution, has been developed by the University of Liberec and commercialized by Elmarco, S.R.O. under the mark Nanospider®. The Nanospider® improves throughput relative to other electrospinning methods, but it is not currently possible to manufacture core-sheath fibers using the Nanospider®. There is, accordingly, a need for a mechanically simple, high-throughput means of manufacturing core-sheath fibers.